3-Way Switch Problem

Need some expert advice, I had a 3-way switch that quit working and am trying to diagnose the issue.

Details:

- There are 2 banks (we'll call them bank A and B) of switches on either end of my kitchen. Each bank contains 3, 3-way switches all controlling the same group of lights (overhead, undercabinet and ambient).

- It was set up so that you could turn on or off any of the three lights from either bank.

- New wiring was installed 3 years ago by an electrician and has worked flawlessly until yesterday. He switched jobs and I no longer have his contact info, otherwise I'd go straight to the source.

- The switch for the overhead light on bank A won't turn ON the lights but, if you've already turned them on with the other switch from bank B, it will turn them OFF. That same switch on bank B is functioning fine. All other switches are working as expected.

My question is what would cause one 3-way switch to begin functioning incorrectly? Faulty switch? Any suggestions?